Urbanization is one of the dominant demographic trends of our time. In 1900, 150 million people lived in cities. By 2000, it was 2.9 billion people, a 19-fold increase. By 2007 more than half of us will live in cities—making us, for the first time, an urban species.

In 1900 there were only a handful of cities with a million people. Today 408 cities have at least that many inhabitants. And there are 20 megacities with 10 million or more residents. Tokyo’s population of 35 million exceeds that of Canada. Mexico City’s population of 19 million is nearly equal to that of Australia. New York, São Paulo, Mumbai (formerly Bombay), Delhi, Calcutta, Buenos Aires, and Shanghai follow close behind.

Cities require a concentration of food, water, energy, and materials that nature cannot provide. Concentrating these masses of materials and then dispersing them in the form of garbage, sewage, and as pollutants in air and water is challenging city managers everywhere.

Most of today’s cities are not healthy places to live. Urban air everywhere is polluted. Typically centered on the automobile and no longer bicycle- or pedestrian-friendly, cities deprive people of needed exercise, creating an imbalance between caloric intake and caloric expenditures. As a result, obesity is reaching epidemic proportions in cities in developing as well as industrial countries. With more than 1 billion people overweight worldwide, epidemiologists now see this as a public health threat of historic proportions—a growing source of heart disease, high blood pressure, diabetes, and a higher incidence of several forms of cancer.

The evolution of modern cities is tied to advances in transport, initially for ships and trains, but it was the internal combustion engine combined with cheap oil that provided the mobility of people and freight that fueled the phenomenal urban growth of the twentieth century. As the world urbanized, energy use climbed.

Early cities relied on food and water from the surrounding countryside, but today cities often depend on distant sources even for such basic amenities. Los Angeles, for example, draws much of its water supply from the Colorado River, some 970 kilometers (600 miles) away. Mexico City’s burgeoning population, living at 3,000 meters, must now depend on the costly pumping of water from 150 kilometers away and must lift it a kilometer or more to augment its inadequate water supplies. Beijing is planning to draw water from the Yangtze River basin nearly 1,500 kilometers away.

Food comes from even greater distances, as is illustrated by Tokyo. While Tokyo still depends for its rice on the highly productive farmers in Japan, with their land carefully protected by government policy, its wheat comes largely from the Great Plains of North America and from Australia. Its corn supply comes largely from the U.S. Midwest. Soybeans come from the U.S. Midwest and the Brazilian cerrado.

The oil that provides much of the energy to move resources into and out of cities itself often comes from distant oil fields. Rising oil prices will affect cities, but they will affect even more the suburbs that many cities have spawned.

It is widely assumed that urbanization will continue. But this is not necessarily so. The growing scarcity of water and the high cost of the energy invested in transporting water over long distances may itself begin to constrain urban growth. For example, some 400 cities in China are already facing a chronic shortage of water.

Against this backdrop, Richard Register, author of Ecocities: Building Cities in Balance with Nature, says it is time to fundamentally rethink the design of cities. Cities should be designed for people, not for cars. He goes even further, talking about pedestrian cities—communities designed so that people do not need cars because they can walk to most of the places they need to go or take public transportation.

Register also says that a city should be seen as a functioning system not in terms of its parts but in terms of its whole. He makes a convincing case that cities should be integrated into local ecosystems rather than imposed on them.

He describes with pride an after-the-fact integration into the local ecosystem of San Luis Obispo, a California town of 50,000 north of Los Angeles: “[It] has a beautiful creek restoration project with several streets and through-building passageways lined with shops that connect to the town’s main commercial street, and people love it. Before closing a street, turning a small parking lot into a park, restoring the creek and making the main street easily accessible to the ‘nature’ corridor, that is, the creek, the downtown had a 40% vacancy rate in the storefronts; and now it has zero. Of course it’s popular. You sit at your restaurant by the creek…where fresh breezes rustle the trees in a world undisturbed by car noise and blazing exhaust.” San Luis Obispo is surrounded by both natural and agricultural landscapes.

For Register, the design of the city and its buildings become a part of the local landscape, capitalizing on the local ecology. For example, buildings are designed to be heated and cooled by nature as much as possible. Cities can largely live on recycled water that is cleaned and used again and again. The “flush and forget” water system will become too costly for many water-short cities in a world after oil. Urban food production, particularly fresh fruits and vegetables, will expand in vacant lots and on rooftops as oil prices rise.

In the years ahead, urbanization could slow or even be reversed. In a world of land, water, and energy scarcity, the value of each resource may increase substantially, shifting the terms of trade between the countryside and cities. Ever since the beginning of the Industrial Revolution, the terms of trade have favored cities because they control capital and technology, the scarce resources. But if land and water become the scarcest resources, then those in rural areas who control them may sometimes have the upper hand. With a new economy based on renewable energy, a disproportionate share of that energy, particularly wind energy and biofuels, will come from nearby rural areas.